(1) Field of the invention
This invention relates to a method for operating the process for polymerizing olefins in a vapor phase process. More particularly, the invention relates to a method for starting the operation so as to reduce the formation of sheet-like polymer and to avoid the unstable reaction in the initial stage of the process of polymerization or copolymerization of .alpha.-olefins in a vapor phase fluidized bed.
(2) Description of Prior Art
When the polymerization of .alpha.-olefins is carried out in a vapor phase fluidized bed, the formation of sheet-like polymer is liable to occur in the initial stage of the polymerization and the sheet-like polymer blocks up the outlet for polymer product and other parts such as pipings in the downstream side. The blocking sometimes makes the operation substantially impossible to be continued.
The formation of sheet-like polymer is liable to occur during the period from the feeding of a catalyst into a reaction vessel before the polymerization to the stage in which a certain quantity of polymer is produced. The sheet-like polymer is seldom formed during the regular or steady-state reaction after the initial state.
Accordingly, it is considered that the formation of sheet-like polymer is caused by the differences in some conditions between:(a) the period between the initial starting stage and (b) the conditions in the regular reaction state of the polymerization system.
In the initial stage of polymerization, it is undesirable that not only is sheet-like polymer formed but also that unstable polymerization conditions are caused to occur. Especially, the bulk density of the polymer obtained in the initial stage is low as compared with the bulk density of the product obtained in the regular reaction state.
In the polymerization using a vapor phase fluidized bed, one of the factors which influences productivity is the bulk density of polymer produced. The productivity is determined by the weight of polymer produced per unit time with a reactor of a certain volume. Therefore, when the bulk density of the polymer is increased, the productivity can be improved. Accordingly, it is desirable to maintain the same higher bulk density from the initial stage to and through the regular or steady state period of the polymerization.
Furthermore, polymer products are taken out from a reactor intermittently and the volume of polymer to be taken out each time is predetermined. In the case that the bulk density of a polymer is low, the quantity of polymer particles in the discharged gases is small. Therefore, the volume of the entrained gases discharged together with the polymer of a certain quantity is increased. The entrained gases consist of unreacted gases including nitrogen and ethylene. It is not advantageous in view of economy to recover the ethylene by separating it from said unreacted gases. Therefore, if it is possible to make the bulk density of polymer product in the initial stage as large as the value in the regular reaction state, the quantity of the above-mentioned entrained unreacted gases can be reduced and the process can be improved in view of economy and production efficiency.
As described above, the productivity and economy can be improved by avoiding the lowering of the bulk density of polymer produced in the initial stage of polymerization.
Furthermore, even when hydrogen gas is fed in a predetermined gas ratio in the initial stage of polymerization, the melt flow rate (MFR) of polymer is sometimes different from the MFR value of the polymer obtained in the regular or steady-state period. In other words, it is observed that the function of hydrogen to regulate the molecular weight is abnormal.
When such a phenomenon is caused to occur, the production of a polymer having predetermined properties is substantially difficult. In such a case, trial and error operation must be repeated by changing the gas composition, measuring the changed value in MFR of obtained polymer and feeding the value back to the gas composition. In the case of a vapor phase fluidized bed, however, it takes many hours for all the polymer to be replaced by a new polymer within a reaction vessel because the residence time of the polymer particles is generally as long as several hours.
Accordingly, if the value of MFR can be maintained at a normal value in the initial stage of polymerization, the normal state production can be started producing neither off-standard product nor second-grade product.